Method of producing surface layers resistant to wear



J. MULLER Feb. 24,1959) METHOD OF PRODUCING SURFACE LAYERS RESISTANT TO WEAR Filed Oct. 7. 1957 Q PRK QlNhkwgs IN VENT OR Jdf/fl/V/VELS M02451? ATTORNEY- United States Patent METHOD OF PRODUCING SURFACE LAYERS RESISTANT TO WEAR Johannes Miiller, Frankfurt am Main, Germany, assiguor to Deutsche Goldund Silber-Scheldeanstalt vormals Roessler, Frankfurt am Main, Germany Application October 7, 1957, Serial No; 688,712 Claims priority, application Germany October 6, 1956 5 Claims. (Cl. 117--127) The present invention relates to an improved process for the production of wear resistant surface layers upon metals, particularly upon ferrous metals.

It is known that the wear resistance of steels can be improved by surface treatments in fused salt baths and solutions. For example, a nitriding and carburizing ef feet is obtained with fused salt baths containing cyanides and cyanates. Also sulfur is introduced into the surface of the metal by treatment of the metal in a boiling sulfide containing solution or by dipping the metal in a reducing fused salt bath containing sulfur in a less than hexavalent form. The disadvantage of these'known procedures is that long periods of treatment are required to obtain layers of sufiicient thickness.

It is an object of the present invention to provide a process whereby wear resistant surface layers can be obtained upon metals, especially, upon iron and iron alloys in a considerably shorter period of time than was heretofore possible.

According to the invention it was unexpectedly found that surface layers providing a surprisingly great resistance to wear and furthermore a substantially lower coefficient of friction than the untreated surfaces can be easily produced in a comparatively short time by treating the workpiece in question in-a melt containing selenium and/or tellurium. It is immaterial whether the selenium and tellurium are in elemental form or in the form of their alkali metal compounds or as salts. The treatment of the work pieces is most simply carried out in a fused salt bath composed of the active selenium and/or tellurium compound, an inert heat transfer agent, for example, of alkali metal and/ or alkaline earth metal compounds such as chlorides, carbonates and hydroxides with reducing additions, such as, cyanide and cyanate to prevent scaling. A content of less than 5% of selenium or tellurium suflices to guarantee the production of the desired surface layers. Preferably the content of selenium and/or tellurium in the fused salt bath is believed between about 0.5% and 1.5%. The effectiveness of the baths depends upon their temperature. The best results are obtained at temperatures between 450 and 600 C. It is advisable to employ a temperature of about 550 C.

It was found that the treatment according to the invention causes a penetration of bound selenium, or respectively, tellurium into the surface. The resulting surface layers are the cause of the high resistance to wear and low coefficient of friction as compared to untreated steel. The process according to the invention, therefore, is advantageously employed for treating workpieces which in use are subject to friction such as, for example, cam shafts, gear wheels, bushings, bearing boxes, running surfaces of shafts, piston rings, working surfaces of cylinders or workpieces used in cutting operations, such as drills, milling cutters, cutting steel and the like. Especially good results are obtained when all parts of an apparatus which work together under frictional contact are treated according to the invention. The treatment 2,875,095 Patented Feb. 24, 1959 2 according to the invention causes no or only a slight increase in the surface hardness of the pieces treated. Consequently, the process according to the invention is not analogous to a nitriding or carburizing treatment.

Several embodiments of the process according to the invention are given in the following examples- Example I A fused salt bath of the composition:

30% potassium cyanate 30% sodium cyanide 0.1% elemental selenium Remainder alkali metal chloride and carbonates in the proportion of 1:1.

was maintained at a temperature of 550 C. and steel Work pieces (C15: normal steel with 0.15% carbon) were treated therein for 60 minutes. Tests showed'that,

Example ll Steel work pieces were treated in the same manner as in Example I with analogous results in a fused salt bath of the following composition:

30% potassium cyanate 30% sodium cyanide 1% sodium selenite I Remainder alkali metal chlorides, carbonates and oxide hydroxides, containing 10% Na CO 1% hydroxide and l oxide, remainder sodium chloride.

' Example III Steelworkpieces were treated in the same manner as in Example I with analogous results in of the following composition:

50% potassium cyanate 30% sodium cyanide 1% sodium selenite Remainder alkali metal chlorides, carbonates and oxideshydroxides, containing ca. 1% hydroxide, ca. 1% oxide, 10% potassium chloride, remainder sodium carbonate.

Example IV Steel workpieces were treated in the same manner as in Example I with analogous results in a fused salt bath of the following composition:

0.5% sodium tellenate 20% sodium cyanide 15% potassium cyanate 10% barium chloride Remainder alkali metal chlorides, carbonates, and oxideshydroxides, containing chloride and carbonate in the proportion of 1:1 and ca. 2% oxide/hydroxide.

The compositions of the baths employed for the treatment according to the invention are controlled and a reduction of the content in active selenium or tellurium is prevented by addition of the corresponding compound. Also the cyanate and cyanide losses are corrected by cyanide additions.

Investigations have shown that the treatment accord ing to the invention leads to the formation of two surface layers, namely a thin upper layer which is relatively easily rubbed off and therefore substantially reduces sliding friction in a manner similar to a lubricant, and a very firmly adhering wear resistant layer therebelow.

The two layered surfaces resulting from the process a fused salt bath acc'or'ding't'o tlie'inventionprovide substantial advantages for certain branches of industry. For example the'breakin period for machine parts treated according to the invention can be materially reduced. Seizing is almost con plet'el'yjavoided and the "so-called emergency run properties of bearings and thelike are substantially improved.

The accompanying drawing shows a diagram comparing the abrasive losses from'surfaces treated according -to the invention and from untreated surfaces.

As can be seen from curve I, representing the abrasive loss for the untreated surface, a substantially constant high abrasive loss per unit of time occurs from the untreated surface, which'is only somewhat lower during a short period at the beginning of the tests.

The abrasive loss per unit of time from surfaces treated according to the invention (represented by curve 11) on the other hand, is the highest at the beginning of the tests because of the upper more easily rubbed oif layer produced thereon and then gradually decreases and finally almost approximates zero. v

The process according to the invention is carried out for instance with copper and alloys rich in ra per as bronze, and aluminum, and particularly alloys containing iron.

It has been found that structural and tool steels are especially suited for a treatment according to the invention. Tests have shown that the lowest economic limit of the active compound was observed at a percentage of 0.005 whilst a remarkable effectwas already obtained with a percentage of 0.05-0.1. In using a percentage of 0.5 through 1.5, excellentresults were obtained with respect to the rapidity as well as to the quality of the layers.

The fused salt bath according to the inventionshould therefore contain at least preferably 15% of cyanate, whilst the best results could be obtained with a quantity of 25 through 45% of cyanate. The presence of cyanide is not indispensable for the aforementioned effect; economically, however, an addition of about of cyanide is advisable.

Tests have shown that the upper limit is about 60% of 'eyansrs and 50% of cyanide; when adding increased quantities losses in substance will be rather important.

The composition of the remainder substances in a salt bathis of no'importance, as long as the fused salt bath I hydroxides will be found i'nth'e fused salt baths according to the invention if these baths have been operated over a period of some hours.

What is claimed is:-

1. A method of producing wear resistant surfaces having a low coefficient of friction uponmetals which comprises treating such metals in a fused salt bath containing about 10% to 60% of cyanate, up to of cyanide and 0.05 to 5% of an active component selected from the group consisting of selenium and tellurium at a temperature between 450 and 600 C.

2. A'metho'd of producing Wear resistant surfaces having a low coefficient of friction upon ferrous metals which comprises treating such metals in a fused salt bath containing about 10% to of cyanate, up to 50% of cyanide and 0.05 to 5% of an active component selected from the group consisting of selenium and tellurium at a temperature between 450 and 600 C.

3. The process of claim 2 in which any remainder of said fused salt bath is essentially composed of a mixture of carbonate and chloride salts.

4. The process of claim 2 in which said salt bath is maintained at atern'perature of about 550 C. during such treatment.

5. A method of producing wear resistant surfaces having a low coefficient of friction upon metals which comprises treating such metals in a fused salt bath essentially composed of about :10% to 60% of alkali metal cyanate, 20 to 50% of alkali metal cyanide, 0.05 to 5% of an active component selected from the group consisting of selenium and tellurium and any remainder essentially a mixture of alkali metal chloride and alkali metal carbonate at 'a temperature between 450 and 600 C.

No references cited. 

1. A METHOD OF PRODUCING WEAR RESISTANT SURFACES HAVING A LOW COEFFICIENT OF FRICTION UPON METALS WHICH COMPRISES TREATING SUCH METALS IN A FUSED SALT BATH CONTAINING ABOUT 10% TO 60% OF CYANATE, UP TO 50% OF CYANIDE AND 0.05 TO 5% OF AN ACTIVE COMPOUND SELECTED FROM THE GROUP CONSISTING OF SELENIUM AND TELLURIUM AT A TEMPERATURE BETWEEN 450 AND 600*C. 